Blood platelets help disguise cancer from the immune system by suppressing T cells, report scientists at the Medical University of South Carolina (MUSC) in the May 5, 2017 issue of Science Immunology. In extensive preclinical tests, a promising T cell therapy more successfully boosted immunity against melanoma when common antiplatelet drugs such as aspirin were added. Zihai Li, M.D., Ph.D., senior author on the article, is chair of the MUSC Department of Microbiology and Immunology, the program leader for the Cancer Immunology Research Program at MUSC Hollings Cancer Center, and the SmartState Sally Abney Rose Chair in Stem Cell Biology & Therapy. Li studies how tumors hide themselves from the immune system.
Li's team found that platelets release a molecule that suppresses the activity of cancer-fighting T cells. That molecule, unsurprisingly, was TGF-beta, which has been recognized for decades for its role in cancer growth. Yet this study is the first of its kind. Most TGF-beta is inactive. Li and his group found that the surface of platelets has a protein called GARP, a molecular hook that is uniquely able to trap and activate TGF-beta. Platelets, which are small cell fragments that circulate throughout the blood and are normally involved in clotting, become the major source of activated TGF-beta that invading tumor cells use to suppress T cells. In other words, platelets help give tumors their invisibility cloak from the immune system. Scientists have known for several years that certain cancers suppress T cells to avoid the immune system. That is why adoptive T cell therapy is one of the most promising advances in modern cancer treatment. It is a type of immunotherapy that awakens the immune system by retraining a patient's T cells to recognize their cancer. T cells are isolated from a patient's blood and retrained, or "primed," to recognize tumor cells. They are then injected back into the patient's bloodstream where they can now hunt and fight cancer.